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F150 Underdrive Pulleys Explained

By:  Connor MC  / Jun 20 2019
F150 Underdrive Pulleys Explained

Underdrive pulleys are a well-known enhancement in the Mustang world, but do those same benefits transfer over and play nicely with an F150 whose use is entirely different than that of a muscle car?

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When it comes to most performance mods it is important to make as efficient of modifications as possible due to the reality that a couple of horses will be lost at the wheel. Fortunately, upgrading you underdrive pulls is a great way to counteract this problem and take your gains off the dyno sheet and on to the real road. Take back the power your truck deserves with a proper underdrive pulley kit and get the most your build has to offer.

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The Concept Behind Underdrive Pulleys

Underdrive pulleys do not add horsepower in the sense that a better set of cylinder heads, supercharger or turbocharger would. Instead, there is less loss between horsepower at the crank and horsepower at the wheel, making them effectively a horsepower restoring device. No matter what the technicalities are or what voodoo magic goes on behind the scene, more horsepower at the wheel is always a good thing – don’t let anyone tell you otherwise.

Underdrive pulleys work by slowing down the spin rate of the belt driven accessories. Accessories that are spinning at a lower speed require less effort from the engine at any given engine RPM, thereby reducing the amount of power the engine wastes just to turn those accessories. The way the underdrive pulleys achieve these different spin rates is simple – they play with the ratio sizes.

Does the Size of My Truck's Pully Matter?

If you have two pulleys, identical in size, placed horizontally and connected by a belt, they will both turn at the exact same rate. For every rotation the left pulley makes, the right pulley turns through the same amount. If you vary the size of the left pulley, however, it will also change the rate at which it turns in relation to the right pulley. If the left pulley is made smaller, it may rotate several times for each complete rotation of the right pulley. To put this into better context, consider below.

For example, take a 2003 F150 with a 4.6L V8 engine. The water pump, alternator, air conditioning compressor and power steering pump are all connected to (via a serpentine belt) and driven by the crankshaft pulley. If the diameter of the crankshaft pulley were decreased, it would cause all the other pulleys to slow down for any given engine RPM, effectively making them underdriven. Turning at a lower speed is less parasitic draw, and therefore more power is transferred to the pavement. Alternatively, we could keep the crankshaft pulley the same diameter, and increase the diameter of all the accessory pulleys. This would produce the same underdriven effect. Thus, an underdrive pulley kit is a kit of specially designed pulley ratios that are intended to reduce parasitic loss.

Will an Underdrive Pulley System Work on My F150?

Presently, the only underdrive pulleys on the market for F150 pickup trucks are for the older 4.6L and 5.4L V8 mod motor pickups. There aren’t any big name kits for the Coyote or EcoBoost family of engines and likely the reason for this is these engines have already been optimized from the factory. Considering that accessories are now mostly being converted to run on electricity (power steering, for example), there is already less parasitic drag to begin with. Some off-brand names are available for the new truck engines as well, but I would be leery of their quality in terms of balancing and proper sizing – they may slow the accessories to such a degree that electrical charging, for example, could be a problem at idle.

What Kind of Horsepower Gains Can I Expect?

Mod motor F150 owners can expect a gain of 7-12 horsepower at the crank and the same amount in torque. Whilst not overly explosive, these gains are noticeable when putting around town or passing on the freeway.

Furthermore, underdrive pulleys are often made from billet aluminum and polished to a mirror-like finish. No extra ponies to be had here, but they do look nice under the hood and will add some bling to the engine bay.

Will Underdrive Pulleys Produce Any Negative Effects?

If you search the crevices of the internet, there are certainly many rumblings that underdrive pulleys can negatively affect accessory performance, particularly at low RPM and idle. The accessories are turning slower, and for example, the alternator, may charge at a reduced rate at idle. It is theorized that conventional hydraulic power steering systems may be harder to operate at low engine speed, or the engine may overheat due to less coolant being circulated due to a slower turning water pump. The key word here is “theorized”. Real world application has shown that there is no discernible difference at low engine speed or idle – all the accessories continue to work properly without negative impact. The only scenario that perhaps a difference could be noted is when an F150 is being underdriven with a high electrical load. Something like a massive stereo system, or powering a large trailer, could see a big enough drop in charging ability at idle. That said, if ever in doubt, a quick call to the tech department of the underdrive pulley manufacturer with a description of your truck and intended driving habits should answer once and for all any doubts or concerns you may have.

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